The present invention relates to a method for the preparation of fine hollow glass spheres coated with titanium dioxide having high mechanical strengths and excellent whiteness along with a photocatalytic activity from a volcanic vitreous deposit sand as a starting material.
By virtue of the very low bulk density and excellent heat resistance, fine hollow glass spheres are highlighted in recent years as a light-weight filling material in various kinds of metallic materials, ceramics, concretes, plastics and the like. On the other hand, titanium dioxide is highlighted as a filler having photocatalytic activity in coating compositions, e.g., paints, plastics and the like. Accordingly, the demands for these materials are rapidly growing in recent years for those applications.
It is known that fine hollow glass spheres can be prepared from certain volcanic vitreous deposit sands. Japanese Patent Publication 48-17645 discloses that the so-called Shirasu sand, which is a volcanic vitreous deposit occurring in southern Kyushu Island of Japan, can be used as a starting material of fine hollow glass spheres by a method in which the Shirasu sand is calcined at a temperature in the range from 800 to 1200.degree. C. for 10 to 600 seconds followed by density classification in water or in air. This method, however, is not suitable when the desired particle diameter of the product spheres is extremely small even by using the volcanic vitreous deposit sand having a particle diameter not exceeding 20 .mu.m as the starting material.
Several proposals and attempts have been made for upgrading the quality of such fine hollow glass spheres. For example, Japanese Patent Publication 4-296750 and Japanese Patent Kokai 8-208272 each disclose a pretreatment of the starting material by using a hot aqueous acid solution or by using a hot aqueous solution containing aluminum sulfate and urea, respectively. Japanese Patent Kokai 9-263425 proposes a method in which the starting material is dispersed in an aqueous solution containing aluminum sulfate and then a precipitant is added to the aqueous dispersion at room temperature.
It may be an idea that fine hollow glass spheres could be imparted with photocatalytic activity by coating the particles with titanium dioxide. A possible process for the preparation of such titanium oxide-coated hollow glass spheres would be that the fine hollow glass spheres prepared beforehand by the above described methods are subjected to a coating treatment with titanium dioxide followed by a second heating treatment to stabilize the coating layer. This method is naturally far from efficient and can hardly be undertaken in industry.